MODULATIONAL INSTABILITY OF QUASI-PLANE-WAVE OPTICAL BEAMS BIASED IN PHOTOREFRACTIVE CRYSTALS

The modulational instability of quasi-plane-wave optical beams in bias ed photorefractive media is investigated under steady-state conditions . The spatial. growth rate of the sideband perturbations is obtained b y globally treating the space-charge field. Our analysis indicates tha t the growth rates depend on the strength of the externally applied el ectric field and, moreover, on the ratio of the optical beam's intensi ty to that of the dark irradiance. Our results are then compared to pr evious local treatments of the space-charge field equation. The two ap proaches are found to be in good agreement in the low spatial-frequenc y regime provided that the external bias field is sufficiently high. C onversely, in the high spatial-frequency region notable differences ma y exist. Relevant examples are provided.